During the last years, the world of Nuclear Medicine has experienced a number of times severe shortages of radioisotopes for different diagnostic procedures. The most prominent of these radioisotopes is Molybdenum-99 (Mo-99) which is used as a precursor for Tc-99m. This latter isotope is used in more than 80% of nuclear imaging tests for detecting cancer, heart disease and other medical conditions. Each day, hospitals and clinics around the world use Mo-99/Tc-99m in more than 60,000 diagnostic procedures.
As for now, the state-of-the-art technology for producing the most important radioisotopes for nuclear medicine (such as Mo-99, I-131, I-125, Xe-133) is based on irradiation of highly enriched uranium (HEU) targets in dedicated nuclear reactors. More than 95% of the present world production of Mo-99 employs neutron fission (n,f) process. It uses an intense thermal neutron beam from a nuclear reactor irradiating a HEU (U-235) target thus producing Mo-99 in 6.161% of all fission events according to the following reaction:U-235+n=Mo-99+Sn13x+ν*n  (Eq. 1)
The irradiation of 1 g of U-235 target for 7 days in a typical thermal neutron flux of 7*1013 n/cm2/s results in approximately 140 Ci of Mo-99 with very high specific activity of more than 104 Ci/g Mo. However, it should be pointed out that the Mo-99 production from the neutron fission (n,f) of U-235 requires very elaborate and very expensive processing facilities. In addition, extreme precautions must be taken to avoid contamination of the Mo-99 with highly toxic fission products and transuranics. This results in high capital investment and running costs, which, in turn, yields in the high cost of producing 1 Ci (n,f) fission Mo-99 being more than four times higher than the cost of 1 Ci of Mo-99 by other methods.
In addition, this approach suffers from two main global problems. The first is that all such five nuclear reactors (one in Canada, three in Europe, and one in South Africa) producing together roughly 90% of the global Mo-99 requirements are very old (“geriatric”) reactors with an average age of 47 years. As a result, these reactors are frequently shut down for unscheduled and time-consuming repair and routine maintenance and, in any case, all of them are close to total decommissioning. The second problem is that the US administration recently began to oppose vigorously the use of HEU for production of radioisotopes because its use endangers the Nuclear Non-Proliferation Treaty (NPT) and nuclear safety in general.
As of now, there is no generally accepted scientific and technological strategy to exit this crisis. One of the proposals mentioned recently is to check the possibility of using a photo-fission (γ,f) reaction by means of a high-power electron linear accelerator instead of thermal neutron fission in a nuclear reactor. In other words, this method relates to electron accelerator production of Mo-99 via the (γ,f) reaction on uranium target instead of the (n,f) reaction in nuclear reactors. In the case of photo-fission, there is no need in HEU since the natural or, at the most, low enriched uranium (LEU) can be used for this purpose. The Mo-99 producing reaction, in this case, can be summarized by the reaction below:U-238+γ=Mo-99+Sn13x+ν*n  (Eq. 2)
The other possibility is based on the photo-neutron, i.e. (γ,n), process in which the heaviest stable isotope of molybdenum, Mo-100 (isotopic abundance of 9.63%), has been irradiated by bremsstrahlung photons from an electron linear accelerator target. The Mo-99 producing reaction, in this case, can be summarized by the reaction belowMo-100+γ=Mo-99+n  (Eq. 3)
Both in the case of the (γ,f) and (γ,n) reactions, the source of gamma radiation is a linear accelerator of electrons with an energy up to 50 MeV and an electron beam power up to 500 kW. The target of such accelerator, which converts the kinetic energy of an accelerated electron beam into bremsstrahlung (braking radiation) should be chosen from the high atomic number (Z) metals such as 73Ta, 74W, depleted U, in order to maximize the bremsstrahlung yield. In such a case, a target to be irradiated, the isotope Mo-100 (for production of radioisotope Mo-99/Tc-99m) has to be attached to the source of the bremsstrahlung target (converter) assembly as close as possible. However, because of the low efficiency of bremsstrahlung production and because of the considerable self absorption of the produced bremsstrahlung radiation in high-Z body of the bremsstrahlung target, this target must effectively be cooled down by distilled water under pressure. All this does increase the distance between the bremsstrahlung source and the sample to be irradiated (Mo-100) and significantly decreases the yield of Mo-99 production. Techniques and apparatus for the production of radioisotopes can be found for instance in the following publications:
U.S. Pat. No. 5,784,423 relates to the production of radioisotopes by exposing a targeted isotope in a target material to a high energy photon beam to isotopically convert the targeted isotope. In particular, the invention is used to produce Mo-99 from Mo-100.
U.S. Pat. No. 5,802,439 relates to the production of 99mTc compositions from 99Mo-containing materials.
The art has so far failed to provide an efficient method and system to overcome the aforementioned drawbacks of the prior art. It is therefore an object of the present invention to provide an apparatus for producing Mo-99 radioisotope.
It is a further object of the invention to provide a method for the production of Mo-99 radioisotope.
These and other objects of the invention will become apparent as the description proceeds.